Conventionally concentrated winding motor used for compressors and the like of air conditioners and refrigerators are configured as shown in FIGS. 10 to 12.
The concentrated winding motor is composed of a stator 60 and a rotor 80 which is rotatively supported in the stator 60. The stator 60 shown in FIGS. 10 and 12 is constituted of laminated stator cores 61. The stator core 61 is formed of teeth 62 provided with windings and a substantially annular yoke 64 for connecting the outer circumferences of the teeth 62. Tooth tip protrusions 65, which protrude in the circumferential direction along the inside diameter of the stator core, are formed on the tips of the teeth 62.
In FIG. 10, the windings provided on the six teeth 62 on the stator core 61 are represented by three-phase windings 63U, 63V, and 63W. The dimensions and shapes of the three-phase windings 63U, 63V, and 63W are not correctly indicated. Correct dimensions and shapes are shown in FIG. 11 where winding is performed.
FIG. 11 is a sectional view taken along line X-X′ of FIG. 10. Windings 63 representing the three-phase windings 63U, 63V, and 63W are provided on the teeth 62 of the stator core via an insulating material 67 composed of an insulator which is formed into a film or is resin molded.
The three-phase windings 63U, 63V, and 63W are star connected with one another and have 120° rectangular wave conduction in which two of three phases are energized and driven at the same time. Further, an applied voltage is changed by PWM control.
Moreover, the stator cores 61 of this shape are laminated straight in the axial direction without forming a skew thereon. Notches 66 formed on the outer circumference of the stator core 61 act as through-holes between a shell 90 and the stator core 61 in the state in which the stator 60 is shrinkage-fitted to the shell 90 of a compressor. The notches 66 also act as passages for a refrigerant.
In the stator 60, the rotor 80 is rotatively held concentrically with respect to the stator 60. The rotor 80 has permanent magnets 82 embedded in a rotor core 81. End plates (not shown) are attached to both ends of the rotor core 81 and a rivet is passed through a through-hole provided in the rotor core, so that the end plates on both ends are fixed. Moreover, a shaft is provided in a shaft hole 83.
Therefore, due to a rotating magnetic field generated by current applied to the three-phase windings 63U, 63V, and 63W which are provided on the stator 60, the rotor 80 is rotated about the shaft by torque which is a combination of magnet torque and reluctance torque.
As described above, in the case of the stator 60 in which a lamination is made straight without forming a skew, attracting or repelling stresses increase between the adjacent tooth tip protrusions 65, thereby increasing vibrations as compared with distributed winding. Such vibrations are considerably affected by a vibration in the radius direction as well as a vibration in a rotating direction.
Particularly in PWM control and 120° rectangular wave conduction in which only two of three phases are energized, vibrations are considerably increased. This is because current flowing into windings contains more harmonics as compared with sine wave driving. Further, in 120° rectangular wave conduction, a rapid change in current generates strong exciting force on the tooth tip protrusions, thereby increasing vibrations.
It is conventionally known that the formation of a skew is effective as a method for reducing variations in torque to have fewer vibrations. A skew is not formed only on the stator. For example, Japanese Patent Laid-Open No. 2000-175380 discloses that a skew is formed on a rotor or on both of a stator and the rotor. Vibrations generated in the concentrated winding motor of FIG. 10 can be reduced by forming a skew.
However, in the case where variations in torque is reduced by forming a skew on a stator and a stator core has ribs forming teeth on an annular inside diameter forming a yoke, the ribs are tilted by the skew. The occurrence of circular vibrations can be reduced but cannot be completely eliminated only by skewing the stator. Further, it is found that windings stored in a noncontact manner in the same winding groove cause a mass with respect to generated slight circular vibration and thus vibrations are increased.